The poster that I replied to was clearly talking about legal and illegal migration. They said “allowing hundreds of thousands of migrants” which is clearly a reference to the overall net migration figures and not the much smaller small boats crossing number.
I think it’s also worth saying that illegal is not the same as small boats. Most illegal immigrants enter the country legally, not in small boats. Unlike many countries we don’t check when people leave the country, so are not good at picking up on people who have entered legally but overstayed.